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Full text of "Text Book Of Mechanical Engineering"

k

l'j!                    1048                            Appendix VL

wheel.    An endless pulling chain lies in the sprocket j, which is
,;Ģi                      keyed to the worm spindle; and the velocity ratio in the example

shewn is about 34 : i; for there are 34 teeth in the worm wheel
and two threads on the worm, making 17:1, and the radii of j and
D are as 2:1; so the total ratio is 17 x 2 = 34 : i. Taking an
efficiency of 75 %, the real mechanical advantage is 34 x '75 = 25*5,
and a pull of 65 Ibs. by one man will lift 25*5 x 65 = 1660 Ibs. or
nearly f of a ton. Referring to the large section in Fig. 942, the
thrust of the worm is received in two places, firstly between the
collar x on the worm spindle and the steel bush T} and secondly
between the bush T and the hard steel pivot u. The former
causes the ratchet socket K to bear against two wedges at R,
which separate the gun-metal segments s s, and compel them to
bind,on the interior of K with a grip proportionate to the weight
lifted. As the segments are held to the worm spindle by keys
v v on a square on the spindle, the overhaul on the worm is
evidently prevented by this frictional grip and the resistance of
the pawl w, shewn in lower view; but if the workman desires to
lower the weight he can do so by pulling on the lowering side of
the chain on j, when the segments s s will slip, the work of over-
coming their friction being proportional to the load itself. Thus
a light load can be lowered several feet by one vigorous pull on
the hand chain, but a heavier load is proportionately resistant.
Now, although the frictional resistance is always present, and
immediately ready to exert itself towards safety, it is entirely
removed on the raising of the load : for the parts E, s, K, and T
then revolve as one, the ratchet wheel clicking freely over the
pawl; and the only resistance is then the friction of the pivot u,
whose diameter is seen to be small.